I couldn't find any mention of that problem in any of my references, and I've never had the problem myself. Is the area around the split brown or sunken? That could indicate a disease problem, and there are a few that attack sunflowers (though they usually cause the plants to wilt). Is there any sawdust-like material around the split, or on the ground underneath it? That could be a sign of borers--these moth larvae tunnel into stems and munching along the way. Try to pry the split open and look for a grub.

If none of these sounds like the problem, my next guess would be something environmental. Heavy rains after a drought or overfertilizing can lead to rapid, and possibly weak, growth. A strong wind might cause the plant to sway, splitting the stem.